Water table
Cavern
Warm ash
and
lava flows
Heat flow
A.
B.
C.
Outflow
Steam
Steam
Heat flow
Geyser
eruption
Empty
chambers
Heat flow
Steam
T
I
M
E
253
Hot Springs and Geysers
diversity, and spectacular nature of Yellowstone’ s
geysers and other thermal features undoubtedly
were the primary reason for its becoming the first
national park in the United States. Geysers are
also found in other parts of the world, notably
New Zealand and Iceland. In fact, the Icelandic
word geysa, “to gush,” gives us the name geyser.
Geysers occur where extensive underground
chambers exist within hot igneous rocks. How
they operate is shown in FIGURE 10.19. As relatively
cool groundwater enters the chambers, it is heated
by the surrounding rock. At the bottom of the chambers, the water is under great pressure because of the
weight of the overlying water. This great pressure prevents the water from boiling at the normal surface temperature of 100 °C (212 °F).
For example, water at the bottom of a 300-meter (1000foot) water-filled chamber must reach nearly 230 °C (450 °F)
to boil. The heating causes the water to expand, with the result
that some is forced out at the surface. This loss of water reduces
the pressure on the remaining water in the chamber, which lowers the boiling point. A portion of the water deep within the chamber quickly turns to steam, and the geyser erupts. Following the
eruption, cool groundwater again seeps into the chamber, and the
cycle begins anew.
When groundwater from hot springs and geysers flows out at the
surface, material in solution is often precipitated, producing an accumulation of chemical sedimentary
rock. The material deposited at
any given place commonly
reflects the chemical make-up
of the rock through which the
water circulated. When the
water contains dissolved silica,
a material called siliceous sinter or
geyserite is deposited around the
spring. When the water contains
dissolved calcium carbonate, a
form of limestone called travertine
or calcareous tufa is deposited.
The latter term is used if the
material is spongy and porous.
D I D Y O U K N O W ?
Many people think that Old Faithful erupts so
reliably—every hour on the hour—that you
can set your watch by it. So goes the legend,
but it’s not true. Time spans between
eruptions vary from about 65 minutes to
more than 90 minutes and have generally
increased over the years thanks to changes in
the geyser’s plumbing.
FIGURE 10.19 Idealized diagrams of a geyser. A
geyser can form if the heat is not distributed by
convection. A. In this figure, the water near the
bottom is heated to near its boiling point. The
boiling point is higher there than at the surface
because the weight of the water above increases
the pressure. B. The water higher in the geyser
system is also heated; therefore, it expands and
flows out at the top, reducing the pressure on
the water at the bottom. C. At the reduced
pressure on the bottom, boiling
occurs. Some of the bottom
water flashes into steam,
and the expanding
steam causes an
eruption.
The deposits at Mammoth Hot Springs
in Yellowstone National Park are more
spectacular than most (FIGURE 10.20).
As the hot water flows upward through
a series of channels and then out at the
surface, the reduced pressure allows
carbon dioxide to separate and escape
from the water. The loss of carbon
dioxide causes the water to become
supersaturated with calcium carbonate,
which then precipitates. In addition
FIGURE 10.18 Old Faithful
geyser in Yellowstone National
Park, Wyoming. (Photo by Jeff
Vanuga/Corbis)
Cavern
Warm ash
and
lava flows
Heat flow
A.
B.
C.
Outflow
Steam
Steam
Heat flow
Geyser
eruption
Empty
chambers
Heat flow
Steam
T
I
M
E
253
Hot Springs and Geysers
diversity, and spectacular nature of Yellowstone’ s
geysers and other thermal features undoubtedly
were the primary reason for its becoming the first
national park in the United States. Geysers are
also found in other parts of the world, notably
New Zealand and Iceland. In fact, the Icelandic
word geysa, “to gush,” gives us the name geyser.
Geysers occur where extensive underground
chambers exist within hot igneous rocks. How
they operate is shown in FIGURE 10.19. As relatively
cool groundwater enters the chambers, it is heated
by the surrounding rock. At the bottom of the chambers, the water is under great pressure because of the
weight of the overlying water. This great pressure prevents the water from boiling at the normal surface temperature of 100 °C (212 °F).
For example, water at the bottom of a 300-meter (1000foot) water-filled chamber must reach nearly 230 °C (450 °F)
to boil. The heating causes the water to expand, with the result
that some is forced out at the surface. This loss of water reduces
the pressure on the remaining water in the chamber, which lowers the boiling point. A portion of the water deep within the chamber quickly turns to steam, and the geyser erupts. Following the
eruption, cool groundwater again seeps into the chamber, and the
cycle begins anew.
When groundwater from hot springs and geysers flows out at the
surface, material in solution is often precipitated, producing an accumulation of chemical sedimentary
rock. The material deposited at
any given place commonly
reflects the chemical make-up
of the rock through which the
water circulated. When the
water contains dissolved silica,
a material called siliceous sinter or
geyserite is deposited around the
spring. When the water contains
dissolved calcium carbonate, a
form of limestone called travertine
or calcareous tufa is deposited.
The latter term is used if the
material is spongy and porous.
D I D Y O U K N O W ?
Many people think that Old Faithful erupts so
reliably—every hour on the hour—that you
can set your watch by it. So goes the legend,
but it’s not true. Time spans between
eruptions vary from about 65 minutes to
more than 90 minutes and have generally
increased over the years thanks to changes in
the geyser’s plumbing.
FIGURE 10.19 Idealized diagrams of a geyser. A
geyser can form if the heat is not distributed by
convection. A. In this figure, the water near the
bottom is heated to near its boiling point. The
boiling point is higher there than at the surface
because the weight of the water above increases
the pressure. B. The water higher in the geyser
system is also heated; therefore, it expands and
flows out at the top, reducing the pressure on
the water at the bottom. C. At the reduced
pressure on the bottom, boiling
occurs. Some of the bottom
water flashes into steam,
and the expanding
steam causes an
eruption.
The deposits at Mammoth Hot Springs
in Yellowstone National Park are more
spectacular than most (FIGURE 10.20).
As the hot water flows upward through
a series of channels and then out at the
surface, the reduced pressure allows
carbon dioxide to separate and escape
from the water. The loss of carbon
dioxide causes the water to become
supersaturated with calcium carbonate,
which then precipitates. In addition
FIGURE 10.18 Old Faithful
geyser in Yellowstone National
Park, Wyoming. (Photo by Jeff
Vanuga/Corbis)